The top of the columns of Devil's Postpile - Columnar Basalt in the Eastern Sierra (California)
A basalt volcanic flow formed hexagonal-shaped columns when it cooled down.
You can clearly see the parallel grooves showing the erosion caused by debris pushed by the slow-moving ice of the glacier that used to cover this valley. Those marks are called glacial striations.
For more info about this rock formation, go to en.wikipedia.org/wiki/Devils_Postpile_National_Monument.